1. Field of the Invention
The present invention relates to a serial scan type ink jet printing apparatus and method.
2. Description of the Related Art
In a serial scan type ink jet printing apparatus of recent years, there are growing demands for faster printing speed. As a method for increasing the printing speed, it is effective to reduce the number of passes in a multipass printing method that is used to enhance a quality of printed images. The number of print passes refers to the number of scans that a print head is required to perform to have one line of image printed. Normally, the ink jet print head is mounted on a carriage which is reciprocally moved in a main scan direction that crosses a sub-scan direction in which a print medium is fed. The ink jet print head has a column of ink ejection openings arrayed in the sub-scan direction. These openings form a plurality of nozzles. An image is progressively formed on a print medium by repetitively alternating a printing scan, by which the print head ejects ink as it travels in the main scan direction, and a feeding operation, by which the print medium is fed a predetermined distance in the sub-scan direction (also referred to as a “paper feed”).
In the multipass printing method, since the print head has a predetermined number of ink ejection openings or nozzles the more the number of passes, the smaller the print medium feed distance (paper feed distance) for each pass will be. Conversely, as the number of passes is reduced, the paper feed distance for each pass increases. Therefore, the printing speed can be increased by reducing the number of passes. For example, when a 4-pass printing is changed to 2-pass printing, the printing speed theoretically increases two times. A 1-pass printing, if applicable, can further enhance the printing speed. That is, as the number of passes decreases, the number of scans that the carriage must perform to complete the printing over a predetermined print area (e.g., print surface of one sheet of print medium) decreases and the paper feed distance per pass increases. This in turn shortens the time required to print one sheet of print medium.
The print head having a plurality of ink ejection nozzles performs a printing scan in a main scan direction almost perpendicular to a direction in which the nozzles are arrayed. Thus, when for example a 2-pass printing is performed, a linear high density area is formed at a boundary between a strip area printed by a first printing scan and a strip area printed by a second printing scan.
In the 2-pass printing, since each band of area is printed by two scans, a duty of ink (relative number of ink dots) applied to the print medium in one scan is about two times greater than that of a 4-pass printing which prints one band of area in four scans. Thus, in a print medium such as plain paper, in which ink dots easily spread, a boundary portion between adjoining band areas which is applied a greater number of ink dots than in other areas has an increased risk of ink spread, although its likelihood varies depending on an ink property. As a result, a dark line (high density line) shows up, degrading the quality of a printed image.
A variety of methods have been proposed to eliminate the above-described dark lines at boundary portions and thereby enhance the quality of printed images (e.g., Japanese Patent Application Laid-open Nos. 2002-36524, 8-25693 (1996) and 7-52465 (1995)).
Japanese Patent Application Laid-open No. 2002-36524 describes a method for the serial scan system which prevents dark lines from being produced at boundary portions between bands of print area when the print head repetitively performs a printing scan, one band at a time, in the main scan direction. That is, in a printing scan that prints on the boundary portion, print data corresponding to an area close to the boundary portion is thinned according to a count value of ink dots formed in that area close to the boundary portion. By thinning the ink dots formed in the area close to the boundary portion, the formation of dark lines can be prevented.
Japanese Patent Application Laid-open No. 8-25693 (1996) describes a method for the serial scan system which makes less noticeable dark lines that are formed at the boundary portions between bands of print area when the print head repetitively performs a printing scan, one band at a time, in the main scan direction. That is, in a 1-pass printing, an image printed in a preceding scan and an image to be printed in the next scan are partly overlapped and a random mask pattern is used for the overlapping image area so that the two scans complement each other in forming ink dots in the overlapping area.
Japanese Patent Application Laid-open No. 7-52465 (1995) describes a method for a serial scan type multipass printing system which makes dark lines formed at the boundary portions less noticeable by randomly setting a paper feed distance using a random number to randomize a dark line occurrence frequency.
Although they can be applied where dark lines are always formed in the boundary portions in image areas, the conventional methods, however, cannot eliminate white lines that are caused by a paper feed accuracy problem and by a phenomenon called an “end nozzle dot deflection.” The “end nozzle dot deflection” is a phenomenon in which ink droplets ejected from end nozzles of a nozzle column in the print head land at positions deviated toward the center of the nozzle column (see Japanese Patent Application Laid-open No. 2003-145775). That is, as ink droplets are ejected from the nozzles, the surrounding air is carried away by the droplets, reducing the pressure of a space near the nozzle face of the print head relative to the surrounding. Thus, the air near the print head flows toward the reduced pressure space. This air flow draws ink droplets, particularly those ejected from the end nozzles of the nozzle column, toward the center of the nozzle column, with the result that the ink droplets land at positions deviated toward the center of the print head This process occurs in the so-called end nozzle dot deflection.
FIG. 1 is an explanatory diagram showing landing positions of ink droplets when no end nozzle dot deflection results In the first scan in FIG. 1, the print head H ejects ink droplets from its nozzles N1-N8 as it moves in the main scan direction indicated by arrow X, to have the ink droplets land on one band of area in the print medium S. Then, the print medium S is fed a distance equal to the width of one band in the sub-scan direction indicated by arrow Y. In FIG. 1, the print head H is depicted as if it was moving in a direction opposite the arrow Y relative to the print medium S. In the second scan the print head H, as in the first scan, ejects ink droplets from its nozzles N1-N8 as it travels in the main scan direction of arrow X, to have the ink droplets land on another band of area in the print medium S. In the case of FIG. 1, the end nozzle dot deflection has not occurred and the ink droplets have landed at intended positions. Therefore, no dark or white line is formed at the boundary portion P in the image area.
However, if an end nozzle dot deflection should occur in which the landing positions of the ink droplets ejected from the end nozzles N1, N2, N6-N8 deviate toward the center of the print head H, the boundary portion P appears as a white line and is easily noticed. Depending on the print head used, the landing positions of ink droplets ejected from end nozzles may deviate toward the outside of the print head. In that case, the boundary portion appears as a dark line. These white or dark lines vary depending on the kind of print medium. In the case of a print medium in which ink easily spreads, such as plain paper, the boundary portions are likely to appear as dark lines.
In the 2-pass printing, since the duty of ink applied to the print medium in one pass is larger than in the 4-pass printing as described above, the amount of the end nozzle dot deflection is larger than that of the 4-pass printing. Thus, when the 2-pass printing is performed on a print medium in which ink dots hardly spread, such as photograph paper, there is an increased risk that the greater end nozzle dot deflection than that of the 4-pass printing can result in more noticeable white lines at the boundary portions, significantly degrading the quality of printed images. If the white or dark lines at the boundary portions are to be made less noticeable by correcting print data corresponding to the end nozzles, as in the conventional techniques described above, the white or dark boundary lines may not be made less noticeable to an expected level and remain to show up. This phenomenon becomes conspicuous as the ink droplets become small (to less than 2.8 pl), a significant hindrance to printing high quality images such as photographs and graphics.